Wide dynamic range (WDR) sensors are used when it is anticipated that the images would comprise details from both low-light scenes and very bright scenes.
Recently new techniques were introduced where the imaging sensor is used to acquire two consecutive images of a badly lit scene, one taken under short exposure conditions whereas the other taken under long exposure conditions. Using image analysis and processing techniques the two images are combined to produce an image with enhanced details.
For example, in US 20040136603 (Vitsnudel et al.), there is disclosed a method for enhancing wide dynamic range in images. The method comprises: acquiring at least two images of a scene to be imaged, the images acquired using different exposure times; constructing for a first image an illumination mask comprising a set of two weight values distinctively identifying respective areas of pixels of high or low illumination, over-exposed or underexposed with respect to a predetermined threshold illumination value, assigning one of the values to each pixels in them, whereas the other value is assigned to other pixels of the other images; using a low-pass filter to smooth border zones between pixels of one value and pixels of the other value, thus assigning weight values in a range between the two weight values; constructing a combined image using image data of pixels of the first image and image data of pixels of the other images proportional to the weight values assigned to each pixel using the illumination mask.
The use of this technique became popular and several imaging sensors commercially available form leading manufacturers are now equipped with the ability to take two sequential exposures—a short one and a long one). These sensors are referred to as WDR sensors, in the context of the present invention, as opposed to “regular” sensors, that sample a single image.
The present invention aims at providing an enhancement of the sensitivity and the robustness of the sampling of WDR sensor in low-light situations where there is no need for short exposure. It is suggested that the high frequency (e.g. 2×—double rate) clock is replaced by low frequency (e.g. 1×—basic rate) clock sampling and processing. By reverting to 1× clock a single long channel is more robustly sampled (sampling duty cycle is increased and exact sampling location is not required) and the noise is reduced.
Alternatively, it is suggested to increase the operating frequency of a regular sensor to get a couple (or more) of images (during one field period of time) that represent different exposures (short and long in the case of two images) of the same scene. It is suggested that the low frequency (e.g. 1×—basic rate) clock is replaced by high frequency (e.g. 2×—double rate) clock sampling, then the images are stored in the memory and subsequently WDR processing is made at the basic rate of 1× . In this case regular sensor might be utilized to handle WDR scenarios.
It is further suggested to separate in a temporal order between the sampling of the CCD and the rest of the imaging system operation. Accordingly, in low-light situations where only one channel is active sampling is done during the first half of the video line, acquired data is stored in a temporary buffer and then processing is performed during the second half of the video line. Thus full decoupling between the sampling and processing is achieved in order to eliminate system noises during the sampling.
Furthermore it is suggested to choose proper sampling times when using a higher frequency clock for additional components in the imaging system, for example memories (e.g. SDRAM). By using high frequency (e.g. 8×) clock and picking a proper subset of smaller number of pulses, which do not intervene with basic (e.g. transitions of 1×) clock sampling it is possible to further decrease the synchronous noise, especially vertical stripes that affect the processed video image.